Behavior maintained by injection of certain drugs in non-human primates serves as an experimental model for drug-seeking behavior in humans. The proposed research would investigate maintenance of long and orderly sequences of behavior leading to injections of narcotics (e.g., morphine and codeine), barbiturates (e.g., pentobarbital) or psychomotor stimulants (e.g., cocaine and d-amphetamine) under conditions where reinforcing effects of these drugs can be dissociated from their other pharmacological effects. Behavior will be maintained in squirrel and rhesus monkeys under second-order schedules where drug is injected only at the end of each experimental session and behavior during the session is maintained by scheduled presentations of a brief stimulus that has been associated with drug injection. In one series of experiments, a range of schedule parameters will be explored and conditioned-reinforcing effects of the brief stimuli will be studied by either eliminating stimulus presentations or altering their duration or frequency. In a second series of experiments, using selected schedule parameters, a full range of i.v. doses of several selected drugs will be studied and the behavior maintained will be compared to behavior maintained under comparable schedules of i.m. drug injection or food delivery. In a third series of experiments, pretreatment with selected drugs (e.g., naloxone, naltrezone, methadone, chlorpromazine, d-amphetamine) will be studied on behavior maintained by injection of cocaine, morphine or pentobarbital or by delivery of food. Some animals will receive large doses of narcotics or barbiturates chronically to assess the effects of physical dependence development on the reinforcing or pretreatment effects of drugs. The overall objective of the research will be to determine the effectiveness of drugs as reinforcers under conditions allowing a dissociation of the reinforcing and other pharmacological effects of a drug and then to evaluate the extent to which the reinforcing effects of drugs can be blocked or modified by environmental or pharmacological manipulations.